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Poster: Diseases of Plants: New & Emerging Diseases


Web blight of spinach in the desert southwest caused by Pythium aphanidermatum
C. FENG (1), B. Liu (1), M. Matheron (2) (1) University of Arkansas, U.S.A.; (2) University of Arizona, U.S.A.

A unique foliar disease of spinach caused by P. aphanidermatum was observed in spinach in the Yuma, AZ/Imperial Valley, CA spinach production areas in October of 2015. The foliar symptoms of the disease included water soaked foliage, rapid collapse of young plants, with evidence of white cottony mycelium. The disease was associated with hot and wet conditions under high density (>3.0 M seed/A) plantings. Pure cultures were isolated from symptomatic spinach leaves and isolates were sequenced using ITS1/ITS4 primers. BLAST searches showed 100% homology with ITS sequences of P. aphanidermatum in GenBank. The objective of this research was to determine the effect of the P. aphanidermatum recovered on damping-off and foliar web blight on a broad range of hosts. Pathogenicity was tested in the greenhouse on spinach, cotton, soybean, pepper, tomato, cucumber, melon, squash, lettuce, corn, wheat, and rice, by either infesting the soil or doing foliar inoculations with colonized plugs of PDA. P. aphanidermatum caused severe damping off of all dicots tested, but only caused some seedling size reduction in corn and wheat. Web blight symptoms were severe with spinach, and all other dicot hosts. No web blight symptoms were observed on lettuce, corn, wheat, or rice. Metalaxyl provided complete protection for damping off of spinach and protected spinach plants for 10-20 days for U.S. treatment rates or 20-30 days after planting for E.U treatment rates, respectively.